Most useful language

Does anyone have an opinion about what the most useful language is for people in the tea business. It seems like Chinese and Japanese would be useful but not sure if one would be a lot more useful than the other.

The answer to your question would rely on your answer to this question:

"What is the focus of your tea store?"

If you are going to specialize in Japanese or Chinese then that is clearly what would be more important. If you wish to be general, then probably Chinese as Chinese would help with many more different varieties of tea than Japanese.